1. Field of the Invention
The invention relates to a device for optical remote monitoring, preferably an optically readable membrane-based microphone, and a system comprising such a device.
2. Description of the Related Art
It is known to use optical techniques for remote monitoring. Prior-art solutions comprise, inter alia, the laser radar technique for vibration analysis. The laser radar is directed to existing objects in the vicinity of the sound that is to be detected. However, it is difficult to achieve a desirable detection level using current methods. This is partly due to the fact that the intensity of the backscattered light from common surfaces is relatively low. In addition, it is not quite certain that the illuminated surface has a strong connection to the surrounding pressure field.
To avoid these problems, it is known to use an optically readable microphone. The microphone is placed close to the sound source so that it can detect the pressure field. The microphone is illuminated and the reflection is processed in order to recreate the sound. U.S. Pat. No. 5,262,884 discloses, for instance, a microphone comprising a membrane with a lens on one side. The light is directed into the lens and the modulated reflection is registered by a detector. The abstract of JP 2190099 discloses another microphone where a mirror is placed on the membrane to improve the reflection of the laser.
However, these solutions do not always provide very clear results. There is also a risk of disturbances and monitoring.